Mysql两个查询结果相加?错误：Every derived table must have its own alias

SELECT username,SUM(ycnum) AS yznum,SUM(zfnum) AS zfnum
FROM
(select * from tempactive where exists 
(select * from topic1activeuser where username=tempactive.username) 
UNION
select * from topic1activeuser where exists 
(select * from tempactive where username=topic1activeuser.username)) 
GROUP BY username

一直报错Every derived table must have its own alias 

提示要给子查询加上别名

SELECT username,SUM(ycnum) AS yznum,SUM(zfnum) AS zfnum
FROM
(select * from tempactive where exists 
(select * from topic1activeuser where username=tempactive.username) 
UNION
select * from topic1activeuser where exists 
(select * from tempactive where username=topic1activeuser.username)) 
AS total
GROUP BY username

结果：

select * from tempactive where exists 
(select * from topic1activeuser where username=tempactive.username)

结果1：

select * from topic1activeuser where exists 
(select * from tempactive where username=topic1activeuser.username)

结果2：